feat(application): implement US1 relay control use cases

Add GetAllRelaysUseCase (T043) for retrieving all 8 relay states with
labels, coordinating controller reads and repository label lookups
with comprehensive error handling and logging.

Implement ToggleRelayUseCase (T041) for toggling individual relay
states with read-before-write pattern, state validation, and label
retrieval.

Add use_cases module (T044) with trait-based dependency injection for
testability, exposing both use cases for presentation layer
integration.

Comprehensive test coverage includes 7 toggle tests (state
transitions, error handling, double-toggle idempotency) and 9 get-all
tests (count, ordering, state correctness, label inclusion, error
scenarios).

Ref: T041 T042 T043 T044 (specs/001-modbus-relay-control/tasks.org)

backend/src/application/use_cases/toggle_relay.rs

@@ -0,0 +1,207 @@
+//! Toggle relay use case.
+//!
+//! This use case handles toggling a single relay's state from on to off or vice versa.
+//! It coordinates with the relay controller to read the current state, toggle it,
+//! and write the new state back.
+
+use std::sync::Arc;
+
+use crate::domain::relay::{
+    controller::{ControllerError, RelayController},
+    entity::Relay,
+    repository::{RelayLabelRepository, RepositoryError},
+    types::RelayId,
+};
+
+/// Error type for toggle relay use case operations.
+#[derive(Debug, thiserror::Error)]
+pub enum ToggleRelayError {
+    /// Error from the relay controller (connection, timeout, protocol issues).
+    #[error("Controller error: {0}")]
+    Controller(#[from] ControllerError),
+
+    /// Error from the label repository.
+    #[error("Repository error: {0}")]
+    Repository(#[from] RepositoryError),
+}
+
+/// Use case for toggling a relay's state.
+///
+/// This use case:
+/// 1. Reads the current state of the specified relay
+/// 2. Toggles the state (On → Off, Off → On)
+/// 3. Writes the new state to the relay
+/// 4. Returns the updated relay entity with its label
+///
+/// # Example
+///
+/// ```rust,ignore
+/// let use_case = ToggleRelayUseCase::new(controller, repository);
+/// let relay = use_case.execute(RelayId::new(1).unwrap()).await?;
+/// // relay.state() is now toggled from its previous value
+/// ```
+pub struct ToggleRelayUseCase {
+    controller: Arc<dyn RelayController>,
+    repository: Arc<dyn RelayLabelRepository>,
+}
+
+impl ToggleRelayUseCase {
+    /// Creates a new toggle relay use case.
+    ///
+    /// # Arguments
+    ///
+    /// * `controller` - The relay controller for hardware communication
+    /// * `repository` - The label repository for relay labels
+    #[must_use]
+    pub fn new(
+        controller: Arc<dyn RelayController>,
+        repository: Arc<dyn RelayLabelRepository>,
+    ) -> Self {
+        Self {
+            controller,
+            repository,
+        }
+    }
+
+    /// Executes the toggle relay use case.
+    ///
+    /// Reads the current state, toggles it, writes the new state, and returns
+    /// the updated relay entity including its label.
+    ///
+    /// # Arguments
+    ///
+    /// * `relay_id` - The ID of the relay to toggle (1-8)
+    ///
+    /// # Returns
+    ///
+    /// The updated `Relay` entity with the new state.
+    ///
+    /// # Errors
+    ///
+    /// Returns `ToggleRelayError` if:
+    /// - Controller fails to read/write relay state
+    /// - Repository fails to retrieve the label
+    pub async fn execute(&self, relay_id: RelayId) -> Result<Relay, ToggleRelayError> {
+        tracing::debug!(target: "use_case::toggle_relay", relay_id = relay_id.as_u8(), "Toggling relay state");
+        let current_state = self.controller.read_relay_state(relay_id).await?;
+        tracing::debug!(target: "use_case::toggle_relay", relay_id = relay_id.as_u8(), ?current_state, "Read current state");
+        let new_state = current_state.toggle();
+        tracing::debug!(target: "use_case::toggle_relay", relay_id = relay_id.as_u8(), ?new_state, "New state after toggle");
+        self.controller
+            .write_relay_state(relay_id, new_state)
+            .await?;
+        tracing::info!(target: "use_case::toggle_relay", relay_id = relay_id.as_u8(), ?new_state, "Successfully toggled relay");
+        let label = self.repository.get_label(relay_id).await?;
+        Ok(Relay::new(relay_id, new_state, label))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::relay::types::RelayState;
+    use crate::infrastructure::modbus::mock_controller::MockRelayController;
+    use crate::infrastructure::persistence::label_repository::MockRelayLabelRepository;
+
+    /// Helper to create a test controller with initialized relays.
+    async fn create_test_controller() -> MockRelayController {
+        let controller = MockRelayController::new();
+        for i in 1..=8 {
+            controller
+                .write_relay_state(RelayId::new(i).unwrap(), RelayState::Off)
+                .await
+                .unwrap();
+        }
+        controller
+    }
+
+    #[tokio::test]
+    async fn test_execute_toggles_relay_state_off_to_on() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let use_case = ToggleRelayUseCase::new(controller.clone(), repository);
+        let relay_id = RelayId::new(1).unwrap();
+        let initial_state = controller.read_relay_state(relay_id).await.unwrap();
+        assert_eq!(initial_state, RelayState::Off);
+        let result = use_case.execute(relay_id).await.unwrap();
+        assert_eq!(result.state(), RelayState::On);
+        assert_eq!(result.id(), relay_id);
+    }
+
+    #[tokio::test]
+    async fn test_execute_toggles_relay_state_on_to_off() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let relay_id = RelayId::new(1).unwrap();
+        controller
+            .write_relay_state(relay_id, RelayState::On)
+            .await
+            .unwrap();
+        let use_case = ToggleRelayUseCase::new(controller.clone(), repository);
+        let result = use_case.execute(relay_id).await.unwrap();
+        assert_eq!(result.state(), RelayState::Off);
+    }
+
+    #[tokio::test]
+    async fn test_execute_returns_error_if_controller_fails() {
+        let controller = Arc::new(MockRelayController::new().with_timeout_simulation());
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let use_case = ToggleRelayUseCase::new(controller, repository);
+        let relay_id = RelayId::new(1).unwrap();
+        let result = use_case.execute(relay_id).await;
+        assert!(result.is_err());
+        assert!(matches!(
+            result.unwrap_err(),
+            ToggleRelayError::Controller(_)
+        ));
+    }
+
+    #[tokio::test]
+    async fn test_execute_updates_state_in_controller() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let use_case = ToggleRelayUseCase::new(controller.clone(), repository);
+        let relay_id = RelayId::new(1).unwrap();
+        use_case.execute(relay_id).await.unwrap();
+        let state_in_controller = controller.read_relay_state(relay_id).await.unwrap();
+        assert_eq!(state_in_controller, RelayState::On);
+    }
+
+    #[tokio::test]
+    async fn test_execute_returns_relay_with_label() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let relay_id = RelayId::new(1).unwrap();
+        let label = crate::domain::relay::types::RelayLabel::new("Test Label".to_string()).unwrap();
+        repository
+            .save_label(relay_id, label.clone())
+            .await
+            .unwrap();
+        let use_case = ToggleRelayUseCase::new(controller, repository);
+        let result = use_case.execute(relay_id).await.unwrap();
+        assert_eq!(result.label(), Some(label));
+    }
+
+    #[tokio::test]
+    async fn test_execute_returns_relay_without_label_when_none_set() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let use_case = ToggleRelayUseCase::new(controller, repository);
+        let relay_id = RelayId::new(1).unwrap();
+        let result = use_case.execute(relay_id).await.unwrap();
+        assert_eq!(result.label(), None);
+    }
+
+    #[tokio::test]
+    async fn test_execute_double_toggle_returns_to_original_state() {
+        let controller = Arc::new(create_test_controller().await);
+        let repository = Arc::new(MockRelayLabelRepository::new());
+        let use_case = ToggleRelayUseCase::new(controller.clone(), repository);
+        let relay_id = RelayId::new(1).unwrap();
+        let initial_state = controller.read_relay_state(relay_id).await.unwrap();
+        assert_eq!(initial_state, RelayState::Off);
+        use_case.execute(relay_id).await.unwrap();
+        let result = use_case.execute(relay_id).await.unwrap();
+        assert_eq!(result.state(), RelayState::Off);
+    }
+}